1. Field of the Invention
The present invention relates to instant film units, also called self-processing film units, and more specifically to such units that include a small access port extending from the exterior of the film unit to its interior for introducing a low viscosity processing fluid into a space between two layers of the film unit.
2. Brief Description of the Prior Art
Instant film units typically include two superposed sheets separated by a spacer. One of the sheets includes a layer of photosensitive material suitable for recording a latent image that is processable by a fluid to form a visible image. The other sheet, frequently called a cover sheet, confines the fluid and facilitates its distribution over the photosensitive material. The spacer controls the depth of the distributed fluid and thereby determines the quantity of the fluid that is available to the photosensitive material for processing.
The processing fluid is usually introduced into the space between the sheets sometime after the film unit is exposed. The latent image is formed in the layer of photosensitive material by exposing the film unit to a scene, and then the fluid is introduced to process the latent image and form the visible image.
Numerous approaches have been employed for storing the processing fluid prior to exposure and for distributing the fluid over the photosensitive material after exposure. In present commercial film units, for example, the fluid is carried in a rupturable pouch disposed at one end of the film unit where it communicates with the edges of one or both of the sheets. Processing of the film unit is initiated by advancing the unit between a pair of pressure rollers which rupture the pouch, to expel its fluid contents between the sheets, and drive the fluid toward the other end of the film unit to coat the photosensitive material. The fluid includes a thickener that increases its viscosity and improves its spreading characteristics under such conditions.
Although satisfactory for its intended purpose, the use of a rupturable fluid pouch typically increases the size of at least one border of the film unit. Similarly, since the pouch is relatively thick, the film units are not uniform in caliper, making a stack of the film units somewhat difficult to handle in the camera. Still further, the viscous fluid, although it is relatively easy to control, requires undesirably high forces to expel from the pouch and distribute between the sheets.
Another approach disclosed in U.S. Pat. Nos. 2,982,650, issued on May 2, 1961, and 3,069,266, issued on Dec. 18, 1962, relies on capillary forces between the photosensitive sheet and cover sheet to draw a low viscosity processing fluid from a reservoir of the fluid into the space between the sheets. The fluid enters the film unit along the full width of the film unit at one end, and moves toward the opposite end of the unit until it fills the space between the sheets. Such a film unit may have a uniform caliper, and equal borders, but the introduction of the fluid into the space between the sheets is undesirably sensitive to the orientation of the film unit because of gravitational effects. Additionally, fluid flow is induced entirely by capillary action, which may be satisfactory in some applications, but is too slow in others.
Still other approaches that employ non-viscous fluids are disclosed in U.S. Pat. Nos. 3,541,938 issued on Nov. 24, 1970, and 3,352,674 issued on Nov. 14, 1967. The fluids are injected from a syringe or small blister pouch into the space between the sheets of a relatively small instant transparency unit. Insofar as the application of the fluid is concerned, such approaches appear to be orientation insensitive. They rely, however, on an external mechanism for applying pressure to the imaging area of the film unit to distribute the fluid. Although satisfactory for their extended purposes, these approaches are more difficult to apply to the larger formats and require undesirably complex camera mechanisms to manipulate the fluid after it is introduced between the sheets. In addition, the film unit itself must be capable of containing the fluid under pressure and then, presumably, releasing any excess of the fluid after processing.